1. Field of the Invention
The present invention relates to a sheet finisher, an image forming apparatus including the sheet finisher, and a sheet processing method.
2. Description of the Related Art
With the development of multi-functional sheet finishers, sheet finishers with both a side-stitch function and a saddle-stitch function have appeared. In the saddle-stitch function, a set of sheet-like recording mediums (hereinafter, “sheets”) is stapled in the center and the stapled set of sheets is half-folded. Japanese Patent Application Laid-open No. 2001-163519 and Japanese Patent Application Laid-open No. 2001-206629 disclose examples of the sheet finishers with the saddle-stitch function. In most of the sheet finishers with the saddle-stitch function, a folding unit that folds the set of sheets includes at least one pair of rollers called pressure rollers and a plate member called folding plate. More particularly, the folding plate is aligned with a line to be folded of the set of sheets, and inserts the set of sheets into a nip between the pressure rollers. Thus, a crease is made along the line to be folded on the set of sheets with the nip.
Some folding units include a first pair of pressure rollers and a second pair of pressure rollers. The set of sheets is pressed twice with the first pressure rollers and the second pressure rollers, which makes a stronger crease. Japanese Patent No. 3566492 and Japanese Patent Application Laid-open No. 2001-19269 disclose examples of folding units including a plurality of pairs of pressure rollers. However, even when the set of sheets is pressed twice, it is difficult to make a crease strong enough due to a short pressing time and a low pressing force. Because a rotation axis of the pressure rollers runs parallel to a direction perpendicular to a sheet conveying direction, a folded side of the set of sheets is pressed in the nip between the pressure rollers only for a short time. Moreover, because the pressure rollers nip the entire folded side at the same time, the pressing force on the set of sheets is distributed, i.e., the pressing force per unit area is low.
Sheet finishers disclosed in Japanese Patent Application No. 3746472 and Japanese Patent Application Laid-open No. S62-16987 are similar to the sheet finishers disclosed in Japanese Patent Application Laid-open No. 2001-163519 and Japanese Patent Application Laid-open No. 2001-206629, except that the sheet finishers additionally include a slidable pressure roller to make a stronger crease. The slidable pressure roller is arranged near an ejection port downstream of the pressure rollers. Upon receiving the set of sheets from the pressure rollers, the slidable pressure roller re-presses the set of sheets while sliding on the folded side in the direction perpendicular to the sheet conveying direction, i.e., along the line of crease by an operation of a screw. This configuration makes it possible to a stronger crease.
Because the folded side is pressed with the slidable pressure roller sliding in the direction perpendicular to the sheet conveying direction, the pressing force is applied only at one point of the folded side making a contact with the slidable pressure roller at a time. Because the slidable pressure roller slides on the folded side, the entire folded side is pressed with the high pressing force. As a result, the strong crease is made on the set of sheets.
Moreover, because the slidable pressure roller slides by the rotation of the screw, the folded side is pressed with the pressing force high enough and the crease strong enough is made on the set of sheets. This also results in decreasing a thickness of the folded side. However, because the rotation of the screw is used to slide the slidable pressure roller, the sliding speed is slow, which results in low productivity.